1. Field of the Invention
The present invention relates to a thermal flow sensor which employs a thermosensitive resistor (heating resistor) to detect the flow rate of fluids such as gases.
2. Description of the Related Art
A flow sensor of a certain type is known which detects the flow rate of a fluid from a balanced state of a bridge circuit including a thermosensitive resistor disposed in the fluid. An example of such a flow sensor is disclosed in Japanese Utility Model Laid-Open No. 61-108930. Explanations will be given below, with reference to FIGS. 1 and 2, concerning a known thermal air-flow sensor having a thermosensitive resistor which serves as a heating resistor and in which platinum-film resistor elements are formed on a ceramic substrate.
The overall arrangement of the known thermal flow sensor will be described with reference to FIG. 1. As shown in FIG. 1, the sensor has a housing 1 defining the main passage for a fluid, a supporting substrate 2 which is provided at a predetermined position within the housing 1 and which comprises, for instance, a pipe defining a measurement passage, a thermosensitive resistor 3 provided on the supporting substrate 2 for converting a change in its temperature caused by the fluid into a corresponding change in its resistance, and an air temperature sensor 4 also provided on the supporting substrate 2 for detecting the air temperature and thus enabling the correction of any variation in the change in temperature of the fluid. The thermosensitive resistor 3 and the air temperature sensor 4 form a bridge circuit, together with resistors R.sub.1 and R.sub.2.
The bridge circuit has junctions b and f connected to the input terminals of a differential amplifier 101 the output terminal of which is connected to the base of a transistor 102. The emitter of the transistor 102 is connected to one terminal a of the bridge circuit, and its collector is connected to the anode of a DC power source 103. The transistor 102 and the differential amplifier 101 form a control circuit which controls the current supplied to the thermosensitive resistor 3, thereby maintaining the bridge circuit in a balanced state.
Referring to FIG. 2, there is shown a conventional structure for supporting the thermosensitive resistor 3. As shown in FIG. 2, the thermosensitive resistor 3 is supported by the upper portions of a pair of supporting members 5 disposed on the supporting substrate 2. The supporting members 5 comprise an electrical conductor and also serve as electrically connecting leads. The resistor 3 is supported in such a manner that its upper and lower surfaces align with a direction A in which the fluid flows. The electric connection between the thermosensitive resistor 3 and the supporting members 5 is established at engaging portions 6, one of which is hatched in the figure.
With the above-described conventional air-flow sensor, flow rate is detected in the following manner. Before a fluid is allowed to flow, a certain voltage is applied to the bridge circuit to cause the thermosensitive resistor 3 to generate heat. When the fluid, i.e. air, comes into contact with the resistor 3, the temperature of the resistor 3 drops, thereby causing a change in the resistance of the resistor 3. A current I.sub.H increased by an amount corresponding to the change in the resistance of the resistor 3, i.e. to the flow rate of the fluid, is caused to flow through the resistor R.sub.1, thereby bringing the bridge circuit into a balanced state. In this balanced state, the voltages at the junctions b and f are equal. On the basis of the current I.sub.H, the voltage V.sub.O =I.sub.H .times.R.sub.1 at the junction b is extracted as the flow rate of the fluid from a signal outputting portion S. A signal from the signal outputting portion S is fed to a signal processing section, not shown, then processed therein, so as to determine the flow rate of the fluid. Because changes in air temperature cause variations in the change in resistance of the resistor 3, the variations are corrected by the use of the air temperature sensor 4.
In recent years, a know thermal flow sensor such as that described above incorporates, as the thermosensitive resistor 3, a small thermal resistor (e.g., a resistor having a length of about 2 mm, a width of about 0.5 mm and a thickness of about 0.1 mm) which processes a small thermal capacity and a high responsibility, in order to effect detection with a higher ability to follow up with changes in the flow rate of the fluid to be measured. However, if such a thermal resistor is mounted using the known supporting structure shown in FIG. 2, the thermal capacity of the supporting members 5 may be greater than that of the thermosensitive resistor 3, leading to the risk that the measuring sensitivity and the measuring responsibility may be degraded.
In addition, since the mechanical strength of the thermosensitive resistor 3 inevitably drops, the resistor 3 may become broken during mounting onto the structure shown in FIG. 2.